1. Field of the Invention
The present invention relates in general to a method for feeding back detailed channel information from a mobile terminal (e.g., mobile phone, mobile station) to a base station in a wireless communication system.
2. Description of Related Art
In a frequency division duplex communication system, i.e., where the downlink signals (base station to mobile terminal) and uplink signals (mobile terminal to base station) are sent on different frequencies, the base station does not know information about the downlink channel since it does not receive any downlink signals. However, the base station could benefit from knowing this downlink information (e.g., amplitude and phase information for some or all of the channel taps of the communication channels) that is measured by the mobile terminal. In particular, the base station can use this channel information to adapt (e.g., power, frequency, modulation scheme, coding) data signals that are subsequently transmitted to the mobile phone. In this way, the base station can maximize the throughput to the mobile terminal. Accordingly, the capacity of the wireless communication system can be significantly increased by using a feedback channel to convey the detailed channel information from the mobile terminal to the base station. However, the quantity of channel information that needs to be fed back to the base station can take up too much bandwidth on the feedback channel between the mobile terminal and base station.
Today, the problem of feeding back such a large amount of detailed channel information can be solved by using a zero-memory quantization method. In the zero-memory quantization method, the real and imaginary parts of each channel coefficient of a channel tap of the channel are quantized to N levels and represented by [log2(N)] bits. Unfortunately, this method still consumes a large amount of bandwidth on the feedback channel. For example in a three-tap channel, the quantizing of real and imaginary parts to 16 levels requires sending [log2(16)]×2×3=24 bits per time unit.
Another way of solving the problem of feeding back such a large amount of detailed channel information is described in PCT Patent Application Ser. No. PCT/SE03/02039 filed on Dec. 19, 2003 and entitled “Adaptive Channel Measurement Reporting”. The contents of this PCT Patent Application are hereby incorporated herein. Essentially, the PCT Patent Application describes various methods of compressing channel measurement reports, feeding back full or incremental reports and methods for varying the accuracy and periodicity of these reports based on the speed of the mobile terminal, the bandwidth and the complexity of the channel. The preferred compression method discussed includes sending only the channel state information for the strongest channel taps to the base station. Although this method generally works well to reduce the load on the feedback channel it still can use too much bandwidth. Accordingly, there is a need for a new method for reducing the number of bits that the mobile terminal needs to send channel information to the base station. This need and other needs are satisfied by the method of the present invention.